According to the present state of the art, in order to calculate the attitude of a vehicle, it is necessary to have two microwave sources in known positions of space. The presence of the GPS constellation realizes this condition and in fact various instruments are today available for such measurements (see references G and H).
Those instruments are designed to provide independent attitude measurements on three axes. The use of gyroscopic instrumentation is optional, and it is not needed for attitude measurement, but only to filter high frequency noise on measurement outputs. The basic scheme described in the literature (see references A and B) for satellite attitude estimation consists of:
a constellation of N source satellites, each of which broadcasts an electromagnetic signal which makes it possible to perform interferometric measurements; and PA1 a receiver having four antennas (one of which for redundancy) pointed in the same direction. PA1 attitude estimation requires only one source satellite in the field of view even intermittently (more than one satellite improves estimation but is not required; if the residual between interferometer and gyro were made on attitude angles, then at least two satellites would be required); PA1 GPS interferometry can be used with a narrow antenna field of view (typically 20 or 30 degrees for semi-cone angle) around the Nadir direction, avoiding collection of differential phase measurements in the lowest quality phase pattern zones (this makes it possible to have an accuracy less dependent on the satellite configuration and multipath problems); PA1 The particular type of dynamic filtering makes possible an on-line estimate of misalignments and constant phase errors (such a characteristic improves the overall system accuracy; the estimating filter speed of convergence depends on the orbital velocity and this function is particularly suited to LEO satellites, less suited to terrestrial vehicles).
According to the scheme indicated above, the differential phase measurement is performed by two pairs of antennas with respect to all the N satellites in the visibility field (a source satellite is meant to be in the visibility field when the signal coming from the satellite has a satisfactory signal-to-noise ratio). We have therefore a system of 2N equations in the three attitude unknowns. This system is solved by the pseudo-inverse method (least square solution).
A necessary condition for the solution of attitude equations is that there should be at least two source satellites in the visibility field of the antenna network. The accuracy of estimate depends on the differential phase errors due to an unbalancing of the hardware measurement device, or on phase errors caused by mutual interaction among antennas (multipath errors), or on a misalignment of antennas.
The antennas commonly used for this purpose are antennas having a wide field of view (70-80 degrees of visibility cone), which are often not suitable for interferometric measurements in terms of thermal sensitivity, phase measurement stability and coupling with satellite structure. This results in limitations for attitude estimation accuracy for those kind of methods. The following references summarize the state of the art relative to the invention described below.
A. J. K. BROCK: GPS Tensor-GPS receiver for attitude and orbit determination, ION-GPS 95, Palm Springs, Sept. 1995.
B. J. K. BROCK, R. FULLER, S. HUR-DIAZ, J. RODDEN: "GPS Attitude and Orbit Determination for Space", Palo Alto, Calif. 94303.
C. C. E. COHEN: Attitude Determination Using GPS, Ph.D. Dissertation, Dec. 1992, Stanford University.
D. S. J. FUJIKAWA, D. F. ZIMBELMAN: "Spacecraft Attitude Determination by Kalman Filtering of Global Positioning System Signals", Journal of Guidance, Control and Dynamics, 1995.
E. R. FULLER, S. GOMEZ, L. MARRADI, J. RODDEN: "GPS Attitude Determination From Double Difference Differential Phase Measurements", ION-GPS 96, Kansas City, Sept. 1996.
The invention is described now in its essential aspects. The gyro makes it possible to calculate attitude variations but not absolute attitude itself. The basic principle of the invention rests on the fact that interferometric measurements can be correlated with each other, even if made in different moments or along different axes, due to the reconstruction that the gyro is able to do of the relative attitude variations in the various measurement times.
For this purpose, it is necessary to compare gyro output signals with those from an interferometer not at the attitude angle measurement level, but at the interferometric measurement level, performing a more efficient dynamic filtering of measurements.
Dynamic filtering is performed by an observer structure in which residuals are directly computed as differences between interferometric measurements of phase and estimated values.
If the vehicle motion is not inertial, as is the case with Earth-pointing satellites, substantial advantages come from this:
The attitude measurement configuration described here consists of a three-axis gyroscopic sensor and an interferometric sensor having at least three antennas (plus one for redundancy) far apart from each other, a piece of processing software common to the two sensors and working on the raw data provided by the sensors (differential phases and angular velocities). The piece of processing software only needs, as its input, the position occupied by the source satellites and user vehicle and provides, as its output, estimates of the attitude and of the misalignment between the antenna system and the gyro reference system. The use of the gyroscopic instrumentation is similar to the one classically known as "gyrocompassing" and described in reference 1.
The described measurement configuration is capable of working for any kind of vehicle, however performance depends on the user vehicle trajectory and on the visibility of source satellites during the trajectory.
In the case of inertial pointing satellites, there is no chance of observing misalignments and receiver line offsets; then an operative step of calibration of Earth pointing should be taken. For illustrative but not limiting purposes, the description is now limited to the case of Earth pointing LEO satellites. The invention, however, can be applied to any vehicle.